There are a number of various applications where determining the time-of-flight of a signal is required. These applications include laser range finders, ultrasonic level detectors, and ultrasonic flow meters.
In general, a system for determining the time-of-flight (TOF) a signal can take one of two forms: pitch-catch and pulse-echo. FIG. 1 illustrates an example of a pitch-catch TOF measurement system, and FIG. 2 shows an example of a pulse-echo TOF measurement system. In either configuration, a transmit (TX) device 110 transmits a signal at a predetermined time and the signal is then sensed some time later by a receive (RN) device 120. In the pitch-catch system shown in FIG. 1, the signal is transmitted directly from transmit (TX) device 110 to receive (RX) device 120, in the pulse-echo system shown in FIG. 2, the signal is bounced or reflected off an object as it travels its path from (TX) device 110 to receive (RX) device 120. In some embodiments of a pulse-echo system, the signal may be sensed with the same device that transmitted the signal i.e., the TX device and the RX device are the same. The time it takes the signal to travel from the TX device to the RX device is the time-of-flight of the signal. The signal may be an acoustic signal, such as an ultrasonic wave or signal, or an electromagnetic wave or signal, for example a microwave signal, a beam of visible or ultraviolet light,
Achieving a required degree of accuracy in the time-of-flight measurement is both critical and difficult.
Most TOF measurement systems in applications such as radar, sonar or ultrasound rely on time domain data. These systems measure the time of reception of a signal versus a selected reference time.
A simple approach uses threshold levels. With a threshold measurement technique, the measurement system determines the times that the transmitted signal and the received signal each cross a threshold level, and estimates the time-of-flight of the signal as the difference between the times of these two threshold-crossings. However, in many systems this will provide a less than desirable resolution. For example, when a system employs ultrasonic transducers, these transducers are resonant devices with a limited bandwidth, and therefore the signal has an associated envelope with a rise and fall time. Any additive noise in the system could cause false readings by either accelerating or decelerating the time when the threshold crossing occurs. This is commonly referred to as cycle slip. In practice, fluctuations in amplitude due to absorption, noise, and temperature limit the accuracy of such a simple threshold approach to TOF measurement.
A number of techniques have been used to improve the resolution of the TOF measurement. One approach involves modulating the envelope of the transmit signal and demodulating it when received. However, this technique still relies heavily on the amplitude information in the signal, and so it still suffers from some degradation from fluctuations in amplitude due to absorption, noise, and temperature.
To increase accuracy, a “marker” may be embedded in the transmitted signal. This marker could be implemented with amplitude, frequency, or phase modulation of the transmitted signal. The receiver can then detect this feature with improved immunity to Boise and other factors which lead to timing uncertainties which degrade accuracy of the TOF measurement. However, this approach can add complexity and cost to the system.
An alternative approach performs a correlation of the received signal in the frequency domain. However, this approach requires two signals, such as two transmitted signals, or a transmitted signal and a reference signal (theoretical or experimental), and again can add complexity and cost.
What is needed, therefore, is an accurate method of measuring or estimating the time-of-flight of a signal. What is also needed is a system which can accurately measure or estimate the time-of-flight of a signal.